Current mobile communication systems are developing into next-generation mobile communication systems for providing service capable of transmitting and receiving a large amount of data at a high speed. In the wireless channel environment of mobile communication systems, as opposed to the wired channel environment, errors occur due to various causes, such as multipath interference, shadowing, attenuation of electric waves, time-varying noise, interference and fading, etc., which results in data loss. In addition, the available frequency band of a wireless channel is limited.
Therefore, next-generation mobile communication systems must use limited frequency band and power resources for the sake of a high transmission rate and reliable transmission in a channel of a varying environment. Adaptive modulation and antenna diversity are the most important factors allowing such technology. The adaptive modulation scheme can improve frequency efficiency in a wireless channel environment. To use a varying channel while maintaining an instantaneous error rate below a reference value, the adaptive modulation scheme adaptively applies a modulation scheme by adjusting a modulation parameter such as a constellation size and a coding rate. In general, a modulation mode is selected based on a comparison result between some previously determined reference values and received signal intensity. In other words, a transmitter receives state information of a transceiver channel fed back from a receiver. Here, when the channel state is good, the transmitter may use a modulation scheme having a high transmission rate to transmit a signal. On the other hand, when the channel state is not good, the transmitter may use a modulation scheme having a low transmission rate to reduce an error rate.
Meanwhile, diversity techniques are being used to remove instability of communication due to fading. According to an antenna diversity scheme among the diversity techniques, a transceiver has a plurality of antennas to transmit and receive a signal using an antenna having a good channel or through multiple paths. When there are a plurality of receiving antennas, the optimum antenna combining method used as an antenna diversity scheme is the well-known Maximum Ratio Combining (MRC).
In general, receivers may be user terminals, and it may be difficult for the receivers to have multiple antennas. In addition, diversity combining schemes have only been researched separately from the adaptive modulation scheme and never been considered in association with the adaptive modulation scheme. But today, to efficiently use limited resources of wireless channels and improve reliability, it is necessary for a transmitting end to combine a diversity scheme using multiple antennas with the adaptive modulation scheme.